A thermoelectric generator is a device that allows to transform thermal energy into electric energy, thanks to the possibility of exploiting the effect known as Seebeck effect.
The functioning of most of prior art thermoelectric generators is based on the use of a thermoelectric material with a high merit factor ZT, such as bismute telluride and alloys thereof.
Nevertheless, bismute telluride and alloys thereof are scarcely available in nature and as a consequence thermoelectric generators are expensive and hardly suitable for mass production.
Moreover, these materials cannot be easily scaled down because they are incompatible with manufacturing processes used in microelectronic manufacturing industries.
As a consequence, it is not possible to realize thermoelectric generators on a large scale or to satisfy big markets (automotive, consumer electronics) thus hardly in line with development trends such as Internet of Things or Wearable electronics.
In the last decades, thus, attention has been paid to the possibility of realizing miniaturized thermoelectric generators, using process technologies that are typical for MEMS devices, CMOS devices and more in general for ICs.
Thermoelectric generators are composed of a set of thermocouples.
A thermocouple is composed of two electrical conducting materials or, as an alternative, to a p-doped semiconductor and a n-doped semiconductor joined together by means of a connection with a high electrical conductivity, typically realized through a metal.
Thermocouples are generally connected electrically in series and thermally in parallel for obtaining a voltage sufficient for powering an electronic device.
A configuration of a thermocouple may be either in-plane or out-of-plane, depending on whether the heat flux flowing throughout a thermocouple is in parallel to or orthogonal with the plane in which electric current flows.
From a point of view, the fabrication processes of thermoelectric generators, the thermocouples of which are in an in-plane configuration (see FIG. 1 that depicts the configuration of the publication of Y. Van Andel et al.), are simpler than fabrication processes of thermoelectric generators the thermocouples of which are in an out-of-plane configuration.
From another point of view, a drawback of a thermoelectric generator the thermocouples of which are in an in-plane configuration in respect to a thermoelectric generator the thermocouples of which are in an out-of-plane configuration, is that area occupation is larger and as a consequence there is a miniaturization loss. In the industry of microelectronics, generally, larger areas imply a greater production cost of the thermoelectric generator.
Examples of miniaturized thermoelectric generators in an in-plane configuration are disclosed in the following publications:                Y. Van Andel, M. Jambunathan, R. J. M. Vullers, V. Leonov, Membrane-less in-plane bulk-micromachined thermopiles for energy harvesting, Microelectronic Engineering, Vol. 87 (2010) 1294-1296;        Xie, J.; Lee, C.; Feng, H. Design, fabrication and characterization of CMOS MEMS-based thermoelectric power generators. J. Micromech. Syst. 2010, 19, 317-324;        Kao, P.-H.; Shih, P.-J.; Dai, C.-L.; Liu, M.-C. Fabrication and characterization of CMOS-MEMS thermoelectric micro generators. Sensors 2010, 10, 1315-1325;        Wang, Z.; Van Andel, Y.; Jambunathan, M.; Leonov, V.; Elfrink, R.; Vullers, J. M. Characterization of a bulk-micromachined membraneless in-plane thermopile. J. Electron. Mater. 2011, 40, 499-503.13;        patent U.S. Pat. No. 7,875,791 “Method for manufacturing a thermopile on a membrane and a membrane-less thermopile, the thermopile thus obtained and a thermoelectric generator comprising such thermopiles” Vladimir Leonov, Paolo Fiorini, Chris Van Hoof (2011).        
Examples of miniaturized thermoelectric generators in an out-of-plane configuration are disclosed in the following publications:                Bottner H., Nurnus, J.; Schubert, A.; Volkert, F. “New high density micro structured thermogenerators for stand alone sensor systems” in Proceedings of 26th International Conference on Thermoelectrics, 2007. ICT 2007 (3-7 Jun. 2007) Pages 306-309.        M. Strasser, R. Aigner, M. Franosch, G. Wachutka, “Miniaturized thermoelectric generators based on poly-Si and poly-SiGe surface micromachining”, Sensors and Actuators A, Vol. 97-98, 535-542 (2002).        Su, J.; Leonov, V.; Goedbloed, M.; van Andel, Y.; de Nooijer, M.C.; Elfrink, R.; Wang, Z.; Vullers, R. J. A batch process micromachined thermoelectric energy harvester: Fabrication and characterization. J. Micromech. Microeng. 2010, doi: 10.1088/0960-1317/20/10/104005.        
Referring to the miniaturized thermoelectric generator mentioned in the publication by Bottner et al. and shown in FIG. 2, this miniaturized thermoelectric generator is obtained by means of a flip chip bonding technique. Nevertheless, the used materials are of known type, i.e. bismute telluride and alloys thereof, and are deposited onto silicon wafer by sputtering.
Referring to the thermoelectric generator mentioned in the publication by Strasser et al. and shown in FIG. 3, this thermoelectric generator is realized by surface micromachining.
In particular, this thermoelectric generator is composed of two thermoelectric couples realized by BiCMOS technology.
Isolation between the hot part and the cold part of said thermoelectric couples is realized by means of an oxide barrier LOCOS (Local Oxidation of Silicon).
The thermoelectric material is a layer with a thickness of 400 nm in polysilicon partially doped with phosphorous for realizing n-type parts and partially doped with boron for realizing p-type parts.
The presence of bridges of aluminum prevents the formation of p-n junctions that otherwise would be present between adjacent thermoelectric parts.
In order to increase the conversion yield of thermal energy into electric energy numerous cavities in the silicon substrate have been realized.
Referring to the thermoelectric generator mentioned in the publication by Su et al. and shown in FIG. 4, this thermoelectric generator is composed of thermocouples realized in p-type and n-type poly-SiGe, connected among them with aluminum.
In particular, thermocouples of said thermoelectric generator are free standing. The thermoelectric material is substantially shaped as a bridge, because the underlying material has been removed for increasing the temperature difference on the terminals of the thermoelectric material.
The lower part and the upper part of said thermoelectric generator are sealed together by means of the flip-chip bonding technique and by means of an adhesive paste.